Is there anything wrong with them?
If not I wouldn't bother replacing them for the sake of it.
The plug caps are integrated into the plug lead and unfortuantly Honda discontinued the part a long time ago.. but with a retail price of £30.00 for each plug lead and cap, would you be willing to spend £120.00 if you could get them
However, all is not lost.
The NC24 uses resistor plug caps as opposed to Resistor plugs.. hardly anybody I know uses the proper plugs in these bikes and just use resistor plugs.. CR8EH9's instead of C8EH9's. But it'll run with both types in.. just a slightly weaker spark with resistor plugs and resistor caps.
So, you can go out and buy generic plus leads and generic plug caps and make your own :-)
But remember... if your using non-resistor plugs (standard) and you make your own leads.. make sure you buy surpressed plug caps. Otherwise if your using Resistor plugs any type of plug cap will do.
I recommend these guys:
http://www.gsparkplug.com/shop/
The sell plugs / leads / caps and everything you need.
If your stuck give them a call (with your plug lead handy and a meaure) and they'll tell you whats available.
Most of the replacement caps screw into the plug lead and then just buy a meter and cut it down to size :-)
Then other end just pushes into the coil :-)